Multi-function plural motor control for material handling apparatus



March 16, 1965 H. A. GORJANC MULTI-FUNCTION PLURAL MOTOR CONTROL FORMATERIAL HANDLING APPARATUS Flled Aprll 12, 1960 INVENTOR. HENRY A.GORJANC [:0 L BY ATTORNEYS United States Patent 3,174,686 MULTI-FUNCTIONPLURAL MOTQR CONTROL FOR MATERIAL HANDLING APPARATUS Henry A, Gorjanc,Wicklifie, Ohio, assignor to The Cleveland Crane dz Engineering Company,Wiclrliife, Ohio,

a corporation of ()hio Filed Apr. 12, 1960, Ser. No. 21,744 7 Claims.(Cl. Slit-51) This invention relates to material handling apparatusincluding electrically operated cranes, trolleys, carriers and the likehaving a plurality of operational functions such as forward and reversemotion, and/ or up and down hoist operation, etc. More particularly thisinvention is directed to apparatus of the character mentioned includinga control apparatus for effecting control of a plurality of operationsor functions from a remote position through conductors and wherein suchapparatus can be effected without requiring an inordinate amount ofconductors.

It is a principal object of the invention to provide a novel andimproved alternating current electrically powered material handlingapparatus having movable carrier means supplied with alternating currentelectric power from conductor bars and having remote multifunctioncontrol means which is particularly simple and efficient in operationand which utilizes the phase differences of the alternating current toprovide a plurality of function contactor actuating circuits, portionsof which circuits are common to more than one such circuit, whereby anumber of function effecting contactors can be selectively actuated orenergized to produce their respective functions, and which circuitsinclude only one control conductor bar in addition to the powersupplying conductor bars.

Another object of this invention is the provision, in an alternatingcurrent electrically powered material handling apparatus of thecharacter mentioned, of a multi-function control circuit comprising asending portion and a receiving portion connected by conductor bar andcurrent collector means and utilizing the alternating current phasecharacteristics of the electric power to provide rectified or polarizedcontrol current for selectively energizing switching means to completecircuits for energizing electrical actuators, such as, motor means, fromthe alternating current power supply, and whereby as many as fourditfercnt function contractors may be selectively actuated in responseto remote control switches in the sending portion of the circuit.

Other objects and advantages of the invention, not specifically setforth above, will be apparent from the following detailed description ofthe presently preferred embodiment thereof, illustrated in theaccompanying drawing forming a part of this specification, and in which:

P16. 1 is a fragmentary clevational View of an electrified, overhead,monorail, carrier-type material handling apparatus forming part of anexemplary embodiment of the invention; and

FIG. 2 is a diagrammatic illustration of a four function control circuitfor operating the carrier of FIG. 1.

This invention is particularly applicable to material handling apparatushaving carriers in the form of bridge crane trolleys, overhead monorailcranes, overhead monorail carriers, and the like which are electricallyoperated and have motor means for performing a plurality of functions,such as, forward and reverse travel along a runway, bridge or track, andup and down hoist operations.

As one embodiment of the invention, a portion of a material handlingsystem, including a trolley or carrier ll adapted to travel along anoverhead monorail track or rail ll, is shown in FIG. 1. The carrier 1!)is provided with a drive motor 12 for propulsion thereof along the rail11 and a hoist motor 13 for raising or lowering a hook block 14, or thelike, forming a part of a hoist mechanism 15. The system includes aplurality of conductor bars, such as a bar 20, associated with the railstructure ll and adapted to provide current or electric power to thecarrier 10 through a plurality of current collectors, such as thecollector 21, mounted on the carrier and in sliding engagement with theconductor bars.

The four functions previously mentioned, namely forward, reverse, up anddown, are representative of the type of functions that may be effectedthrough the use of the control apparatus or circuit described hereafter,and per units the remote control of the functions through the conductor'bars usually required for providing electric power and a singleadditional conductor bar. The functions are effected in response tooperation of control switches which may be mounted in the cab of abridge type crane for controlling the trolley thereof, or on a wallbracket or the like for controlling a carrier, such as the carrier ll),of a monorail system. In these instances the control switches aremanually operable, however it will be recognized that in automaticdispatch systems, or the like, the control switches may be in the formof limit switches associated with the rail ill and operable by thecarrier 1t) itself. It will also be recognized that the control systemmay be employed to perform operations off the carrier, such as,operation of a track switch.

A four function control circuit is illustrated in FIG. 2 and comprises asending portion 22 and a receiving portion 23. The receiving portion 23is carried by the trolley or carrier it? and is connected to the sendingportion through current collectors 21a, 21b and 210 in engagement withconductor bars Zita, 20b, and 20c forming a part of the sending portion22. The circuit operates on alternating current supplied through powerlines L1 and L2 connected to the power conductor bars Ella, 2th: andincludes a control line C connected to the control conducto-r bar sec.The power lines L1 and L2 may be two lines of a three phase system thethird line which is designated L3 or may be the two lines of a singlephase system.

The sending portion 22 comp-rises four diode type rectifiers 25, 26, 27and 2S and push button switches Bil, 31, 32 and 33 for effecting theaforesaid forward, reverse, up and down functions, respectively. Thesecontrol switches complete circuits, which will be more particularlydescribed hereafter, to energize and actuate a plurality of solenoidactuated relays or function contactors 35, 36, 37 and 38 in thereceiving portion 23 for completing motor energizing circuits of thetype which are well known to those skilled in the art to which theinvention pertains and which operate the motors l2 and 13 to carry outthe listed functions.

The power lines L1 and L2 carry alternating current and exhibit oppositepotentials at each half-cycle thereof, that is, during one half-cyclethe line L1 is positive with respect to L2, and during the otherhalf-cycle L2 is positive with respect to L1. The control line C isadapted to be connected to one or the other of the power lines L1, L2 bydepressing the appropriate push button switch 30, 31, 32 or 33.

The receiving portion of the circuit 23 comprises conductors 40, 41 and42 which are connected to the current collectors 21a, 21b and 21c,respectively, and the conductors 4d and 42 thereof continuously exhibitthe same potentials as the power lines L1 and L2 from which they areenergized. The forward and reverse function relays or contactors 3S andas are connected across the conductors it} and 42 in series relationwith diode type rectificrs 466 and 47, normally closed contacts 48 and49 of the contactors 35 and 36, and a pair of resistors 59 and 51. Theup and down function relays or contactors 37 and 38 respectively, aresimilarly connected across the conductors 4t) and 42, in series relationwith diode type rectifiers 56 and 57, normally closed contacts 58 and 59of the contactors 37 and 38 and a pair of resistors 60 and 61. Theconductor 41 which is connected to the control line C by the currentcollector 21b is connected commonly between the resistors 50, 51 and theresistors 60, 61.

The contactors 35, 36, 37 and 38 are provided with normally opencontacts 62, 63, 64 and 65 respectively, which contacts when closedserve to complete conventional circuits for energizing the motor means12 and 13. The contacts 62 are connected in series circuit between line40 and a line 66 connected to L3 by the conductor 26c and the collector21d with the forward solenoid e7 of the reversing motor controller forthe travel motor 12 and the contacts 63 are connected in series circuitbetween the same lines with the reverse solenoid 68 of the motorcontroller. The contacts 64, 65 are connected to the up and downsolenoids, respectively, of the reversing controller for the hoist motor13 in a similar manner.

The rectifiers as and 47 are oriented so as to conduct current from theconductor 40 to the conductor 42 when the former is positive withrespect to the latter, and to lock current flow when the conductor 42 ispositive with respect to the conductor 49. It will be recognized thatbecause of the above described series relationship of the contactors and36 that there will normally be a rectified or polarized current flowfrom the conductor through the contactors 35 and 36 to the conductor 42and that a potential will appear across each of the contactors of avalue something less than one-half of the potential existing between theconductors 40 and 42. Likewise, there will appear across each of thecontactors 3'7 and 33 a potential which is something less than one-halfof the potential which exists between conductors 42 and 40. Thepotentials across the contactors are less than onehalf because of thevoltage drops introduced by the series connected resistors 51, and 61,and by the rectifiers 4e, 47, 5s and 57.

The contactors 35, 36, 37 and 38 are chosen to have such operatingcharacteristics that they will not be actuated or pick up at half thepotential between conductors 40 and 42, but will be actuated at morethan half of that potential. Therefore, although a small current isnormally flowing through each set of contactors 35, 36, 37 and 38, theywill not be actuated thereby.

In order to energize the drive motor 12 in a sense to propel the carrier10 in a forward direction the push button 39 is depressed, therebyclosing the normally open contacts thereof and opening the normallyclosed contacts 71 thereof. The closing of the contacts 70 completes acircuit which may be traced from the power line L1 through a conductor75, normally closed contacts '76 of the push button switch 31, aconductor 77, the now closed contacts 70, the diode type rectifier 25,to the control line C and the conductor bar Ztlb, the current collector21b, conductor 41, the resistor 51, normally closed contacts 49 of thereverse contactor 36, a diode type rectifier 46 and the actuatingsolenoid 78 of the forward contactor 35 to the conductor 42 connected tothe current collector 210 which is in conducting engagement with theconductor bar 20b connected to the power line L2. The conductor 41 isthereby brought to the same potential as the conductor 40, with theeffect of shorting out the reverse contactor 36, rectifier 4'7 andresistor 50.

The forward function contactor 35 therefore has almost the full linevoltage impressed thereacross and is energized, opening its normallyclosed contacts 48 and closing its normally open contacts 62. The nowclosed contacts 62 are connected in a suitable manner to complete acircuit for energizing the forward solenoid 67 of the motor controllerwhich in turn causes the drive motor 12 to operate in the direction toeffect forward movement of the carrier 10.

It will be recognized that the rectifier 46 will permit current flowtherethrough only during the half-cycle in which conductors 4t 41 arepositive with respect to conductor 42, and therefore a smoothingcapacitor is connected directly across the solenoid 78 of the contactor35. The capacitor 80 is charged during current flow through therectifier 46 and discharges through the solenoid 78 when the rectifierblocks current flow from the conductor 42. The resistance 51 serves toslow the charging of the capacitor 80 and, as mentioned before, toassure that the contactor 35 normally has less than half the linevoltage thereacross and will not pick up unless the control switch 30 isdepressed.

if the forward control switch 39 is now released, opening the contacts76 thereof, the current flowing in the solenoid '78 of contactor 35 willfall to zero because the normally closed contacts 48 thereof are open.With the loss of current thereto the contactor 35 will drop out orrelease its armature to close the contacts 48, and to open the contacts62 for de-energizing the motor 12. The contacts 48 are positionedbetween the conductors 40, 41 because, although the contactor 35 willnot pick up at half potential, there is a possibility that it would notdrop out on a mere reduction of potential from full line voltage to halfpotential as would occur if the contacts 48 were positioned betweenconductors 41 and 42, which would permit current to flow from theconductor 40 through the contactor 36 and resistor 5t) to the contactor35 immediately upon switch 39 being released.

If it is now desired to run the carrier along the rail 11 in a reversedirection, the push button control switch 31 may be depressed, closingthe normally open contacts thereof and opening the normally closedcontacts 76 thereof. Closing of the contacts 85 completes a circuitwhich may be traced from the conductor ill through the solenoid 86 ofthe reverse contactor 36, the rectifier 47, the normally closed contacts48 of the contactor 35, the resistor 5t conductor 4-1 and the currentcollector 21b to the conductor bar 20b, the conductor or control line C,the rectifier 2e and now closed contacts 85 of the control switch 31, aconductor 88, contacts 1 of the forward control switch 30, and aconductor 5.9 to the power line L2.

This circuit shorts out the resistor 51 and the solenoid 78 of theforward contactor 35, thereby increasing the current flow through thesolenoid 36 to a level where the reverse contactor 36 is actuated,drawing in its armature to open the normally closed contacts 4-9 thereofand to close the normally open contacts 63 thereof. The latter, contacts63, serve to complete a circuit for energizing the motor 12 in a senseto propel the carrier 10 in a reverse direction along the rail 11.

A smoothing condenser or capacitor 9% is connected across the solenoid86 and serves to store electricity during the half-cycle during whichcurrent flows to the solenoid 85 and to give up electricity therethroughduring the other half-cycle.

In order to halt the reverse movement of the carrier 10, the switch 31is released, thereby reducing to zero the potential across the contactor36 and causing the contacts 63 thereof to open.

Operation of the up and down functions is accomplished through portionsof the control circuits similar to those described with respect to theforward and reverse functions, and is described hereafter. When it isdesired to raise the hook 14, the up push but-ton or control switch 32is depressed, closing the normally open contacts 92 thereof and openingthe normally closed contacts 93 thereof.

Closing of the contacts 92 completes a circuit which may be traced fromthe conductor 42, through the actuating solenoid 95 of the up contactor37, the rectifier 56, normally closed contacts 59 of the down contactor38,

the resistor 61 and the conductor 41 to the current collector 21b, andfrom the conductor bar 20b through the control line C, the rectifier 27,the now closed contacts 92 of the up control switch 32, a conductor :97,normally closed contacts 98 of the down control switch 33, and aconductor 99 to the power line L1. This circuit serves to short out thedown contactor 38 and the resistor 60, thereby increasing the currentfiow through the solenoid d5 to a level where the armature thereof isdrawn in.

Drawing in of the armature opens the normally closed contacts 53 thereofand closes the normally open contacts 6 thereof. The latter, contacts64, complete a circuit which is effective to energize the hoist motor 13in a sense to operate the hoist mechanism 15 to raise the book block 14.The opening of the contacts 58 prevents the likelihood of the contactor37 remaining drawn in upon releasing the control switch 32 and openingthe contacts 92 thereof.

in order to lower the hook block 14, the push button control switch 33may be depressed, opening the normally closed contacts 98 thereof andclosing the normally open contacts 106 thereof. Closing of the contacts100 effects the completion of a circuit which may be traced from thepower line L2 through a conductor lfil, the normally closed contacts 93of the up control switch 32, a conductor 1%, the now closed contacts 1%of the switch 33, the rectifier 28, the control line C and conductor barZtlb, the current collector 21b, the conductor 41, resistor 6d, contacts58, the rectifier 57, and the actuating solenoid 1&5 of the downcontactor 38 to the conductor 4t). This circuit serves to energize thecontactor 38 by increasing the current flow therethrough, drawing in thearmature thereof to open the normally closed contacts 59 and to closethe normally open contacts 65. The

latter, cont-acts 65, complete a circuit which is effective to energizethe motor 13 in a sense to operate the hoist mechanism for lowering thehook block 14.

Again, it will be noted that the contactors 37 and 38 are provided withcapacitors lit and 111 respectively, for storing up electricity duringthat half-cycle when the conductor 42 is positive with respect to theconductor 4t) and for discharging current through the solenoids 95 and1% during the other half-cycle.

The control switches 30 and 31 are electrically interlocked so thatdepressing of both switches at the same time cannot cause a shortcircuit from the line L1 to the line L2. Likewise, the control switches32 and 33 are electrically interlocked to prevent the possibility ofcreating a short circuit from line L2 to line L1 in the event bothswitches are depressed. The electrical interlock in each case iseffected by providing each switch with normally closed contacts andnormally open contacts, with the normally closed contacts of each switchconnected in series with the normally open contacts of another switch.In the instance described above where the control switch 30 wasdepressed to actuate the forward contactor 35, for example, the normallyclosed contacts '76 of the switch 31 are in series with the contacts 70of the switch Sfi. Had the switch 31 been depressed at the same time asthe switch 36, no circuit would have been completed.

Because the forward and reverse switches 30, 31 are interlocked, and theup and down switches 32, 33 are interlocked, the circuit is ideal forcontrolling reversing motors such as the motors 12 and 13 since each isonly capable of running in one direction at a time.

The motors 12 and 13 can, however, be operated simultaneously withoutregard to direction because of the orientation of the rectifiers inpairs, with the rectifiers 46, 4'7 and 25, 26 arranged for permittingcurrent fiow in one direction and the rectifiers 56, 57 and 27, 28 forpermitting current fiow in the opposite direction.

The invention can be used to perform merely two functions such asforward and reverse, up and down, or the like, and the interlockreferred to omitted and the wiring thus simplified by omitting one orthe other pairs of contactors 35, 36 and 37, 38 and the control elementsassociated therewith including the pairs of switches 30, 31 and 32, 33,respectively.

From the foregoing detailed description of the preferred embodiment ofthe invention, and a modification thereof, it will be recognized thatthere has been provided thereby a particularly economical multi-functionelectrical control systems and/ or material handling system havingimproved control means for effecting a plurality of motor functions orthe like from a remote position and using the power conductors thusrequiring the inclusion of but one additional conductor. The system isparticularly rugged and simple in construction yet flexible andadaptable in use.

Although the invention has been described in considerable detail andwith reference to specific embodiments thereof, it is understood thatthe invention is not limited thereby but rather the invention includessuch changes, modifications and uses thereof as are reasonably embracedby the scope of the claims hereof.

Having described my invention, I claim:

1. In a multi-function control circuit, first and second conductorsadapted to be connected to an alternating current source, a firstcontactor connected in series with a first rectifier means between saidfirst conductor and a third conductor, a second contactor connected inseries with a second rectifier means between said third conductor andsaid second conductor, said contactors connected in series across saidfirst and second conductors drawing insufiicient current to actuate thesame, a first control switch connected between said first conductor andsaid third conductor, and a second control switch connected between saidthird conductor and said second conductor, said first rectifier meansbeing oriented to conduct current in a direction from said firstconductor toward said third conductor, and said second rectifier meansbeing oriented to conduct current in a direction from said thirdconductor toward said second conductor whereby closing of said firstcontrol switch shorts out said first contactor to effect actuation ofsaid second contactor and closing of said second control switch shortsout said second contactor to effect actuation of said first contactor,said first and second control switches each having normally opencontacts and normally closed contacts, the normally closed contacts ofeach control switch being connected in series relation with the normallyopen contacts of the other control switch.

2. In a multi-function control circuit, first and second conductorsadapted to be connected to an alternating current source, a firstcontactor connected in series with a first rectifier means between saidfirst conductor and a third conductor, a second contactor connected inseries with a second rectifier means between said third conductor andsaid second conductor, said contactors connected in series across saidfirst and second conductors drawing insuficient current to actuate thesame, a first control switch connected between said first conductor andsaid third conductor, and a second control switch connected between saidthird conductor and said second conductor, said first rectifier meansbeing oriented to conduct current in a direction from said firstconductor toward said third conductor, and said second rectifier meansbeing oriented to conduct current in a direction from said thirdconductor toward said second conductor whereby closing of said firstcontrol switch shorts out said first contactor to eitect actuation ofsaid second contactor and closing of said second control switch shortsout said second contactor to effect actuation of said first contactor,and means to prevent simultaneously establishing a circuit through saidfirst and second control switches.

3. In a multi-function control circuit; first and second powerconductors adapted to be connected to an alternating current source; acontrol conductor; a first contactor and a first rectifier connected inseries relation between said first conductor and said control conductor;a second contactor and a second rectifier connected in series relationbetween said control conductor and said second conductor; a thirdcontactor and a third rectifier connected in series relation betweensaid first conductor and said control conductor; a fourth contactor anda fourth rectifier connected in series relation between said controlconductor and said second conductor; said contactors connected in seriesacross said first and second conductors drawing insufiicient current toactuate the same, a first control switch and a fifth rectifier connectedin series relation between said control conductor and said secondconductor; a second control switch and a sixth rectifier connected inseries relation between said first conductor and said control conductor;a third control switch and a seventh rectifier connected in seriesrelation between said control conductor and said second conductor; and afourth control switch and an eighth rectifier connected in seriesrelation between said first conductor and said control conductor; saidfirst, second, fifth and sixth rectifiers being oriented to pass currentin one direction and said third, fourth, seventh and eighth rectifiersbeing oriented to pass current in the opposite direction wherebyactuation of said control switches will operate said contactorsselectively.

4. A circuit as defined in claim 3 and wherein said first contactorincludes normally closed contacts in series with said second contactor;said second contactor includes normally closed contacts in series withsaid first contactor; said third contactor includes normally closedcontacts in series with said fourth contactor; and said fourth contactorincludes normally closed contacts in series with said third contactor.

5. A circuit as defined in claim 3 and wherein said control switcheseach comprise normally open contacts and normally closed contacts, thenormally open contacts of each control switch being connected in serieswith the normally closed contacts of another of the control switches.

6. In a material handling system including a carrier and having electricmotor means for effecting at least four functions: first, second, third,and fourth solenoid actuated contactors to which said motor means areresponsive to selectively effect said four functions; said system havingalternating current transmitting means including first, second, andthird sets of cooperating relatively fixed and movable conductiveelements; said first contactor being connected in series with a firstrectifier means between a conductive element of said first set and aconductive element of said third set; said second contactor beingconnected in series with a second rectifier means between a conductiveelement of said third set and a conductive element of said secondset;-said third contactor being connected in series with a thirdrectifier means between a conductive element of said first set and aconductive element of said third set; said fourth contactor beingconnected in series with a fourth rectifier means between a conductiveelement of said third set and a conductive element of said second set;said contactors connected in series across said first and secondconductors drawing insufiicient current to actuate the same; a firstcontrol switch and a fifth rectifier means connected in series between aconductive element of said i3 0 first set and a conductive element ofsaid third set; a second control switch connected in series with a sixthrectifier means between a conductive element of said third set and aconductive element of said second set; a third control switch connectedin series with a seventh rectifier means between a conductive element ofsaid first set and a conductive element of said third set; and a fourthcontrol switch connected in series with an eighth rectifier meansbetween a conductive element of said third set and a conductive elementof said second set; said first, second, fifth and sixth rectifier meansbeing oriented to pass current in one direction and said third, fourth,seventh and eighth rectifier means being oriented to pass current in theopposite direction.

7. In a material handling system including a carrier having electricmotor means for effecting at least four functions and having currentcollector means for receiving electric power from conductor bar means:first, second, third and fourth solenoid actuated contactors to whichsaid motor means are responsive to selectively effect said fourfunctions; first and second conductor bars adapted to carry alternatingcurrent; first and second current collectors for engagement with saidfirst and second conductor bars; a third conductor bar and a thirdcurrent collector for engagement therewith; said first contactor beingconnected in series with a first rectifier means between said firstcurrent collector and said third current collector; said secondcontactor being connected in series with a second rectifier meansbetween said third current collector and said second current collector;said third contactor being connected in series with a third rectifiermeans between said first current collector and said third cu"- rentcollector; said fourth contactor being connected in series with a fourthrectifier means between said third current collector and said secondcurrent collector; said contactors connected in series across said firstand second conductors drawing insufficient current to actuate the same;a first control switch and a fifth rectifier means connected in seriesbetween said first conductor bar and said third conductor bar; a secondcontrol switch connected in series with a sixth rectifier means betweensaid third conductor bar and said second conductor bar; a third controlswitch connected in series with a seventh rectifier means between saidfirst conductor bar and said third conductor bar; and a fourth controlswitch connected in series with an eight rectifier means between saidthird conductor bar and said second conductor bar; said first, second,fifth and sixth rectifier means being oriented to pass current in onedirection and said third, fourth, seventh and eighth rectifier meansbeing oriented to pass current in the opposite direction.

References Cited in the file of this patent UNITED STATES PATENTS1,463,168 Kintzing July 31, 1923 2,401,358 Lazar June 4, 1946 2,418,806Reeves et al. Apr. 8, 1947 2,892,132 Mallory June 23, 1959 2,941,638Hoover June 21, 1960 2,943,255 Fresne et al June 28, 1960

1. IN A MULTI-FUNCTION CONTROL CIRCUIT, FIRST AND SECOND CONDUCTORSADAPTED TO BE CONNECTED TO BE ALTERNATING CURRENT SOURCE, A FIRSTCONTACTOR CONNECTED IN SERIED WITH A FIRST RECTIFIER MEANS BETWEEN SAIDFIRST CONDUCTOR AND A THIRD CONDUCTOR, A SECOND CONTACTOR CONNECTED INSERIES WITH A SECOND RECTIFIER MEANS BETWEEN SAID THIRD CONDUCTOR ANDSAID SECOND CONDUCTOR, SAID CONTACTORS CONNECTED IN SERIES ACROSS SAIDFIRST AND SECOND CONDUCTORS DRAWING INSUFFICIENT CURRENT TO ACTUATE THESAME, A FIRST CONTROL SWITCH CONNECTED BETWEEN SAID FIRST CONDUCTOR ANDSAID THIRD CONDUCTOR, AND A SECOND CONTROL SWITCH CONNECTED BETWEEN SAIDTHIRD CONDUCTOR AND SAID SECOND CONDUCTOR, SAID FIRST RECTIFIER MEANSBEING ORIENTED TO CONDUCT CURRENT IN A DIRECTION FROM SAID FIRSTCONDUCTOR TOWARD SAID THIRD CONDUCTOR, AND SAID SECOND RECTIFIER MEANSBEING ORIENTED TO CONDUCT CURRENT IN A DIRECTION FROM SAID THIRDCONDUCTOR TOWARD SAID SECOND CONDUCTOR WHEREBY CLOSING OF SAID FIRSTCONTROL SWITCH SHORTS OUT SAID FIRST CONTACTOR TO EFFECT ACTUATION OFSAID SECOND CONTACTOR AND CLOSING OF SAID SECOND CONTROL SWITCH SHORTSOUT SAID SECOND CONTACTOR TO EFFECT ACTUATION OF SAID FIRST CONTACTOR,SAID FIRST AND SECOND CONTROL SWITCHES EACH HAVING NORMALLY OPENCONTACTS AND NORMALLY CLOSED CONTACTS, THE NORMALLY CLOSED CONTACTS OFEACH CONTROL SWITCH BEING CONNECTED IN SERIES RELATION WITH THE NORMALLYOPEN CONTACTS OF THE OTHER CONTROL SWITCH.